The present invention relates to automatic test equipment for testing and verifying bare workpieces realized as printing wiring boards (PWBs), integrated circuit semiconductor dies (ICs), and the like. The present invention also relates to a method for registration for use with the automatic test equipment.
Bare workpieces are commonly tested with a variety of automatic test equipment which verify different aspects of functionality such as continuity, insulation resistance, detection of undesirable opens and shorts, improper connections, etc. Generally speaking, automatic test equipment can be classified as follows: Automatic Optical Inspection (AOI) systems for inspecting inner layers of workpieces and Ohmic test systems for the final testing of a manufactured workpiece. In other words, the AOI systems and the Ohmic test systems complement one another in the testing of workpieces.
Ohmic test systems can be generally divided into two types of systems: bed of nails systems or flying probe systems.
Bed of nails systems typically employ bulky fixtures with either rigid metallic probes or spring-loaded, gold-plated probes. In both cases, the systems require high static pressure to ensure reliable contact between the probes and test points on the workpieces. Such systems suffer from a number of disadvantages. First, the rigid metallic probes, and even the delicate spring-loaded probes, often leave witness marks, such as notches and cavities, due to the high static pressure which may, in turn, cause a high rejection rate due to the necessity for compliance with strict standards. Second, the high static pressure frequently causes false shorts between adjacent probes especially when the tested board is crowded with fine-pitch shapes. Third, in case that expensive standard fixtures are not used, each workpiece requires a dedicated fixture which imposes a considerable burden on the total manufacturing cost of workpieces, especially in the case of small lots or prototypes. Fourth, test points are sometimes deleted because their locations are difficult or expensive to probe and therefore, in many cases, fixtures do not provide full coverage of a workpiece. Fifth, single terminal nets, such as fudicials cannot be tested by single probe contact. Sixth, the test switching process is done using a large number of plug-in switch cards which reside remote from the test probes, thereby requiring complex electrical cabling.
Flying probe systems employ probes displaceable along the X and Y axes. These systems suffer from the disadvantages that they can only cope with modest-sized lots due to their poor throughput and that moving parts are well known in the industry for their frequent problems.
There is therefore a need for automatic test equipment for verifying different aspects of bare workpieces which overcome the disadvantages of existing automatic test equipment and a method for registration between a test mattress of the automatic test equipment and a bare workpiece placed randomly on the test mattress.